ASTRONOMER

Life
Studied in Cyzicus (Asia Minor) with Ptolemarchus, a friend of Eudoxus, whom he succeeded as head of the school. Later he went to Athens, where he studied with Aristotle. One of the craters on the moon has been named "Callippus" in his honour.


Work
Callippus developed and perfected the system of concentric spheres proposed by Eudoxus, adding seven more spheres - one for each of the planets (Mercury, Venus, Mars, Jupiter and Saturn) and one each for the sun and the moon, both then considered planets - for a total of 34. He made detailed observations of the rising and setting of the fixed stars, especially the "dawn-risers" and "evening-setters". His calculation of the duration of the seasons - Spring 94 days, Summer 92, Autumn 89, Winter 90 - was accurate to within 0.08 - 0.44 days. He improved on Meton's 19-year luni-solar calendar, introducing the 76-year (4 x 19) cycle named after him, which skipped one day every 76 years, and gave a year of 365.25 days and a month of 29.53. His sole known work, "On the System of the Planets", is lost.